In recent years, a communication system has been increased in speed, and various communication apparatuses constituting the communication system have been improved in performance. A bit error rate defined as a comparison between the number of bit errors in received data and a total number of received data is known as one of an indicator of signal quality evaluation in these communication apparatuses.
In a standard such as 200G or 400G defined by IEEE, transmission using a PAM4 signal is defined instead of transmission using a PAM2 (non return to zero (NRZ)) signal so as to respond to an extremely high bit rate. The PAM4 signal is configured to include four PAM4 symbols including “0 (00)”, “1 (01)”, “2 (10)”, and “3 (11)”.
In transmission using a PAM4 signal, precoding is used as an encoding method for removing a burst error. A precode encoder which performs precoding in a transmission apparatus performs a process of converting the PAM4 signal into a precode symbol P(j) according to the following equations (1) and (2).P(j+1)=(G(j+1)−P(j))mod 4   Equation (1)P(1)=G(1)   Equation (2)
Here, j is an integer equal to or more than 1. P(j) is a j-th precode symbol. G (j) is a j-th gray code symbol, and four PAM4 symbols of 0, 1, 2, and 3 in a PAM4 signal are respectively converted to 0, 1, 3, and 2.
On the other hand, a precode decoder which decodes the precoded PAM4 signal in a transmission apparatus performs a process of converting the precode symbol P(j) into a gray code G(j) according to the following equations (3) and (4).G(j+1)=(P(j+1)+P(j)) mod 4   Equation (3)G(1)=P(1)   Equation (4)
Here, j is an integer equal to or more than 1. P(j) is a j-th precode symbol. G(j) is a j-th gray code symbol.
FIG. 5 illustrates an example of a most significant bit (MSB) and a least significant bit (LSB) of the precoded PAM4 symbol output from the precode encoder, and an example of an MSB and an LSB of the gray code output from the precode decoder in a case where an error is added to the PAM4 symbol output from the precode encoder. The hatching area in FIG. 5 indicates a bit or a symbol to which the error is added. In addition, a field of “transition difference” in FIG. 5 illustrates a level difference between the precoded PAM4 symbol before the error is added and the precoded PAM4 symbol after the error is added.
As illustrated in FIG. 5, for example, in a case where an error of 1 bit is added to only an MSB of the precoded PAM4 symbol, an error of 2 bits is generated in a gray code of MSB after decoding. On the other hand, in a case where a burst error of n bits (n≥2) is added to only the MSB of the precoded PAM4 symbol, an error of 2 bits is generated in the gray code of MSB after decoding. In this manner, precoding is a very effective encoding method for removing a burst error added to the MSB.
In the related art, in order to generate a test signal for measuring an error rate in a communication apparatus, an error addition device which adds an error to a data signal is used (for example, see Patent Document 1).